The Roswell Report: Fact vs. Fiction in the New Mexico Desert

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The actual atmospheric distribution, however, does not indicate an adiabatic lapse rate for air but rather a lapse rate which varies with altitude. For the troposphere the lapse rate of the atmosphere averages $ - 1.98^{\circ}\mathrm{C} / 1000 $ ft. It may be shown then that in the troposphere a rising balloon will get warm with respect to the air (neglecting ventilation and radiation effects) at a rate of $ 1.98 - .57 = 1.41^{\circ}\mathrm{C} / 1000 $ ft. In the tropopause the lapse rate of the atmosphere is zero. Thus the lifting gas (if helium) will cool relative to the air at a rate of $ .57^{\circ}\mathrm{C} / 1000 $ ft.

Similarly, in the stratosphere, the lifting gas will cool relative to the air at a rate of 2.24 + .57 = 2.81 $ ^{\circ} \mathrm{C} / 1 0 0 0 $ ft. This effect is plotted as Figure 22.

Figure 22. Lapse rate of air and helium.

Here, below point A, the lifting gas will be warmer than the air. Above point A, the lifting gas will be cooler than the air. The effect of this temperature difference on the lift (as shown in the previous section) is approximately $ \Delta L=1\frac{\Delta T}{1} $

$$
\Delta L = L \frac {\Delta T}{T} \frac {1}{(1 - B)}
$$

Thus, as a balloon system passes through point A, it will have less lift than at release. This effect has been observed on several flights, where a balloon system slowed down during ascent through a temperature inversion.

Since the effect of the sun in heating the lifting gas decreases the effect of different lapse rates, the effect is not as noticeable during the day as at night. At night the balloon system may pass through an inversion, lose its lift, and remain at an altitude much below its estimated floating altitude until warmed by the sun's rays at sunrise.